1. Field of the Invention
This invention relates to the field of measuring characteristics of earth formations penetrated by a borehole and sensing borehole parameters. More particularly, the invention relates to a method of borehole compensation of earth formation characteristic measurements.
2. Description of the Related Art
As a borehole tool positioned in a well borehole passes through various rock layers of the surrounding earth formation, hole rugosity, cave-ins and washouts affect the borehole tool measurements and are potentially a source of measurement error. Misinterpretation of formation characteristics complicates formation evaluation for exploitation of oil and gas resources. Borehole compensation removes the effect of borehole irregularities on certain formation measurements, such as acoustic or electromagnetic measurements.
For example, a majority of wells where logging-while-drilling is applied are either highly deviated or horizontal. Drilling a deviated or horizontal well requires a mud motor with steerable assembly to build the angle of well trajectory when necessary. It is well known that the borehole drilled by such a steerable assembly is a helical or corkscrew borehole with a periodicity of a few feet. The diameter of a helical borehole may vary by a few inches as well. Because the acoustic or electromagnetic measurements of interest are computed by the relative time delay of formation arrival signals between receivers on a borehole tool, the measurement's value is affected by the unequal mud path through which the formation arrival signals pass. Therefore, there is a need in the art to develop new techniques for borehole compensation to provide more accurate formation measurements, especially during exploration drilling operations.